The structure described above of a composite sandwich panel is known, but its production poses problems. In particular, the median layer is difficult to manipulate and to position because of its deformability. A known process consists in deploying a median layer of the dimensions of one panel, so as to form recesses that are as regular as possible, then separately and uniformly cementing the two plates and applying them on opposite sides of the median layer, on the edges of the recesses. The cemented plates are difficult to handle. It is necessary to apply them one after the other on the intermediate layer. There is a large consumption of cement.